Hello there! I wrote today this tiny js script to export my watchlater playlist to a json file in order to import it into piped, as Youtube does not want you to do this for mysterious reasons. Sharing if anybody is in need!
https://codeberg.org/tubbadu/youtube-watchlater-playlist-to-piped-exporter
Disclaimer
The procedure includes pasting js code into the browser’s console. This is usually NOT recommended, as it may expose security vulnerabilities and do really bad things. Use this tool at your own risk. I pinky promise it is not malicious ;)
As long as I know there is no official way to do this, but I think you can pretty easily automate it with selenium